Conventional valves for automatically discharging drain or drainage in compressed air include float type, air pilot type, and motor type valves, among which the motor type is the most reliable in discharging the drain. The conventional drain discharging apparatus of the motor type has a fixed or variable drain discharging cycle or drain discharging period. In the fixed period type, the drain discharging period must be selected to drain the largest probable quantity of the drain to be discharged. Accordingly, in the winter season when the quantity of drain is the smallest, a relative large quantity of purge air is discharged, thus unduly increasing the air loss. In a small air pressure system, a drop in pressure is caused thereby.
In the variable type which is adjustable according to the quantity of the drain produced, the drain discharging period is usually set somewhat longer so as to ensure discharge of the drain and, accordingly, the apparatus is complicated in construction.
Further, absence of an indicator to check the operating condition of the apparatus from the outside is a cause of uncertainty.